You can cancel or downgrade your subscription anytime.
When you downgrade, the change will be effective immediately, and the payment difference will be saved as a credit for the next month.
Instead, when you unsubscribe, you can choose whether you prefer to cancel immediately or to wait until the end of the billing period.
If you are not sure about the date you can access the page dedicated to your sub by clicking on the arrow near your name (top right corner of the page) and then “My subscription”.
In the "Current plan" section you'll find the rebilling date.